Lens driving device for optical read and/or write system and optical read/write system

ABSTRACT

A lens driving device ( 1 ) or an optical read and/or write system comprises a mechanical structure ( 3 ) with an objective lens ( 2 ) and an actuator ( 4, 4′, 6 ) for controlling the lens position. The lens driving device comprises a further actuator ( 5, 5   a,    5   b,    5 ′) on or near the mechanical structure so as to at least partially compensate motion generated by the first-mentioned actuator ( 4,6 ).

FIELD OF THE INVENTION

The invention relates to a lens driving device for an optical readand/or write system, comprising a mechanical structure having anobjective lens, and an actuator for controlling the lens position byacting on the mechanical structure.

The invention also relates to an optical read and/or write systemcomprising a lens driving device comprising a mechanical structurehaving an objective lens, and an actuator for controlling the lensposition by acting on the mechanical structure, the system furthercomprising a controller means for generating a control signal for theactuator, the actuator acting in response to the control signal.

BACKGROUND OF THE INVENTION

Lens driving devices as well as optical read and/or write systemscomprising lens driving devices are known. An optical read and/or writesystem reads information recorded on an optical medium, e.g. on a disk,using laser light to read/write a signal optically and/or writeinformation on said optical medium. The lens driving device for such anoptical read and/or write system drives an objective lens while positioncontrol of the lens, e.g. focus control and tracking control, areexecuted in accordance with the driving signals supplied to drivingactuators, e.g. coils consisting of a focus coil and tracking coil woundon a holder provided with the objective lens. The lens driving devicecomprises a mechanical structure with an objective lens, usually on aholder which is generally suspended by suspension means. Actuators, forinstance tracking and focusing coils on or near the mechanical structuree.g. on or near the lens holder in co-operation with magnets on a fixedpart allow the position of the lens to be controlled, e.g. the lensholder can be moved in a radial direction (tracking) and a verticaldirection (focusing). Alternatively, the device may have coils on afixed part and magnet mechanical structure, e.g. on the lens holder. Thelens driving device generally has respective resonance frequencies inthe focus control and tracking movement, each resonance having a certainmode shape (characteristic movement of the structure at a resonancefrequency). These natural resonance frequencies (eigenfrequencies)depend, inter alia, on the physical shape of the mechanical structure.This shape also determines the anti-resonances, e.g. frequencies wherethe movement of the mechanical structure at the position of the lens isvery small due to cancelling effects of the different mode shapes. Suchnatural resonance and anti-resonance frequencies are typically situatedaround or slightly above 1 to 10 kHz.

In order to follow the tracks on the optical medium as accurately aspossible, the bandwidth of the total system comprising the actuatedmechanical system and a feedback controller must be as large aspossible. However, the combinations of resonances and anti-resonances asdescribed above are a limit to this bandwidth. In the case of theseresonance and anti-resonance combinations, it is not possible inpractice to design a simple (PID or PI-lead/lag) feedback controller,such that the total system has a loop gain that is smaller than 1 forthe frequency where the phase is −180°, while the bandwidth of thissystem is in the region of the resonance/anti-resonance peaks. That is,if the loop gain comes close to −1, the system gets unstable anduncontrollable.

One way of avoiding these problems is to design the mechanical structurein such a way that its natural resonance frequencies lie at very highfrequencies, such that the bandwidth of the controller can reach itsspecifications. The lens driving device is designed so that each highermode resonance is out of each servoband. Namely, by designing theservoband necessary for actual servocontrol at an upper limit of e.g. 2kHz-5 kHz, the control system is unaffected by the phase shift in thevicinity of the natural resonance frequency. EP 1 079 377 discloses adesign aimed at achieving an increase of the natural resonancefrequency. In recent years, however, the disk read and/or write systemshave been operated at a high rotating speed of a disk that is severaltimes the prevailing standard rotating speed of the disk. This increasesthe speed with which a signal is read and/or written by the lens drivingapparatus for the disk player, and it also increases the driving speed,and thereby the driving frequencies of the drive. Thus there is atendency that the upper limit in the servoband of the control systemincreases, leading to a need to increase the natural resonance frequencyof the mechanical structure. This makes it often difficult to reach veryhigh resonance frequencies, because of limitations on the space that canbe occupied by the mechanical structure, or notwithstanding an increaseof the natural resonance frequency, the increase of read/write speedalso increases the upper limit (in frequency) of the servocontrol to afrequency approaching a natural resonance frequency.

OBJECT AND SUMMARY OF THE INVENTION

It is an object of the invention to provide a lens driving device of thetype described in the opening paragraph and an optical read and/or writesystem comprising a lens driving device with improved high frequencycharacteristics to reduce one or more of the indicated problems.

To this end, the lens driving device comprises a further actuator actingon the mechanical structure so as to generate at a frequency range amotion of or in the mechanical structure, to at least partiallycompensate motion generated by the first-mentioned actuator.

To this end, the optical read and/or write system comprises a lensdriving system comprising a further actuator acting on the mechanicalstructure so as to generate at a frequency range a motion of or in themechanical structure, to at least partially compensate motion generatedby the first-mentioned actuator, the controller comprising means forgenerating a compensation signal for said further actuator.

The further actuator excites the mechanical structure at the sameresonances as the first-mentioned actuators to compensate the motioncaused by the first-mentioned actuator. In this manner, the resonancesare actively cancelled, and the harmful oscillations are avoided. Thelens driving system can be operated up to high frequencies.

In a preferred embodiment, the further actuator comprises apiezo-electric element. Within the broadest concept of the invention,the actuators may be e.g. a coil in combination with a magnetic systemor e.g. a piezo-electric element. Use of a piezo-electric element ispreferred because the further actuator is used at relatively highfrequencies (the higher resonance frequencies), for which piezo-electricelements are well suited, and in general the additional weight caused bythe further actuator is preferably small, and the weight ofpiezo-electric elements is generally smaller than the combined weight ofa coil and magnet system. Furthermore, a piezo-electric element isgenerally smaller than an electromagnetic actuator comprising a coil andmagnet system.

These and other aspects of the invention are apparent from and will beelucidated with reference to the embodiments described hereinafter.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings:

FIG. 1 illustrates a scheme for an optical read and/or write system inaccordance with the invention,

FIG. 2 shows in a perspective view an embodiment of a lens drivingdevice in accordance with the invention,

FIG. 3 shows in a perspective view another embodiment of a lens drivingdevice in accordance with the invention,

FIG. 4 shows in a perspective view a further embodiment of a lensdriving device in accordance with the invention,

FIG. 5 shows in a perspective view yet another embodiment of a lensdriving device in accordance with the invention,

FIG. 6 shows a lens driving device in accordance with the invention, and

FIG. 7 illustrates in a graphical form the effects of the invention.

The Figures are not drawn to scale. Generally, identical components aredenoted by the same reference numerals in the Figures.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

FIG. 1 schematically describes some elements of a system in accordancewith the invention. On a mechanical structure 1, a lens is attached to alens holder (not shown in the scheme, see for examples the followingFigures). Attached to or near the mechanical structure is an actuator 4,which receives a control signal CS from a controller (in this example inthe form of a control circuit CC). The input for the controller is asensor output SO, which is fed in this example to a feedback controller(FC). These elements form the basic elements by which the position ofthe lens on the mechanical structure is controlled. However, themechanical structure I has natural resonance frequencies(eigenfrequencies).

In order to follow the tracks on the optical medium as accurately aspossible, the bandwidth of the total system comprising the actuatedmechanical system and a feedback controller must be as large aspossible. However, the combinations of resonances and anti-resonances asdescribed above are a limit to this bandwidth. In the case of theseresonance and anti-resonance combinations, it is not possible inpractice to design a simple (PID or PI-lead/lag) feedback controller,such that the total system has a loop gain that is smaller than 1 forthe frequency where the phase is −180°, while the bandwidth of thissystem is in the region of the resonance/anti-resonance peaks. That is,if the loop gain comes close to −1, the system gets unstable anduncontrollable.

One way of avoiding these problems is to design the mechanical structurein such a way that its natural resonance frequencies lie at very highfrequencies, such that the bandwidth of the controller can reach itsspecifications. However, there is a limit to making the eigenfrequencieshigher, especially in view of the constraints imposed on the design andthe fact that the read/write speed becomes ever higher.

The invention has for its object to solve the above problems in adifferent manner. To this end, the lens driving device comprises afurther actuator on or near the mechanical structure for acting on themechanical structure so as to generate at a frequency range a motion of,or in the mechanical structure, at least partially compensate motiongenerated by the first-mentioned actuator.

A further actuator 5,5 a,5′,5 b is placed on or near the mechanicalstructure. It (they) will excite the mechanical structure at the sameresonance frequencies as the actuator 4. By feeding a compensatingcontroller signal COMPS to the further actuator(s) at a frequency range(to this end filters F may be provided) to the further actuator(s) as isshown in FIG. 1, unwanted resonances can be compensated. A gain G, whichmay be a tunable gain, may be provided to set the gain for thecompensation signal. The gains may be different for differentcompensating actuators. This is schematically indicated in FIG. 1 bygain G′. Preferably, the further actuator is designed in such a way thatit predominantly excites the resonance frequency that is to becancelled.

By compensating the motion of actuator 4, the system remains stable andcontrollable, also when a controller is designed in such a way that thebandwidth of the system is near a resonance frequency of the mechanicalsystem. It is noted that electronically eliminating the problem by usinga notch filter in the control circuit (a filter that is specificallytuned to stop a particular frequency) can also avoid that the systembecomes unstable. However, such notch filters have to be tuned for eachdevice, and furthermore, ageing and temperatures effects may cause intime a mismatch between the eigenfrequency and the frequency of thenotch filter. In the invention, such problems are smaller.

The filters used in the controller may be simple high-pass filters, orbandpass filters.

FIG. 2 shows schematically in a perspective view a lens driving device Iin accordance with the invention. A lens 2 is positioned on a mechanicalstructure 3, in this embodiment a swing arm 3. A force is generated bycoil 4 in the focus direction and by coil 6 in the radial direction. Tosuppress (compensate for) unwanted resonances of mechanical structure 3,an actuator, in this embodiment a thin piezo-electric element 5 isattached to mechanical structure 3. The permanent magnets whichcooperate with the coils in generating the forces are not shown here.The coil may be positioned on the movable mechanical structure, in whichcase a permanent magnet system is positioned on a fixed part of thedevice, or alternatively, the permanent magnet system is attached to themechanical structure, in which case the coils are positioned on a fixedpart of the device. It is preferred, however, that the coils areattached to, fixed to or form part of the mechanical structure 3. Themechanical structure has a relatively smaller weight, which reduces thepower dissipation and increases the resonance frequencies.

FIG. 3. shows a second embodiment. This embodiment comprises the samemechanical structure as shown in FIG. 2, except that the piezo-electricelement 5 is divided into two separate zones 5 a, 5 b. By feeding theseseparate zones 5 a, 5 b through different filters i.e. at differentfrequency ranges (see FIG. 1) and/or by designing them in such a waythat more resonances are excited, more than one resonance can becompensated.

FIG. 4 shows yet a third embodiment, similar to the embodiment shown inFIG. 2, except that focus movement is not generated by a coil 4, but bya thin piezo-electric element 4′, for instance, glued on the bottom ofthe mechanical structure 2. The combination of piezo-electric elements 5and 4′ makes the structure thinner and smaller, which in itself is anadvantage. It is noted that the invention is to be understood to offer aroute for reducing problems with resonances. The invention is not to beso restrictively interpreted as being unable to be combined with othermeasures of reducing problems with resonances. For instance, making themechanical structure thinner and lighter (as in the example of FIG. 4)reduces the weight, thereby reducing power consumption. It may also leadto an increase of the resonance frequency, which is an advantage.

FIG. 5 shows yet a further embodiment of a lens driving device inaccordance with the invention. It comprises the same actuators as in theembodiment shown in FIG. 2, but now the compensating actuator 5′ is anelectromagnetic actuator, comprising a coil placed on top of themechanical structure 3. A permanent magnet system (not shown here) forcooperation with the actuator 5′ is attached to the fixed housing forthe swing arm.

A fifth embodiment is shown in FIG. 6. It comprises a lens 2 on amechanical structure comprising a lens holder 3 a, hinges 3 b and a base3 c. The focusing and radial movements are generated by electromagneticactuators of which only the permanent magnet system 7 and the radialcoils 8 are shown. The resonance of the hinges during the focusingmovement are reduced (compensated) by piezo-elements 9 on top of thehinges, while the resonances during radial movement are suppressed bypiezo-electric elements 10.

Finally, FIGS. 7A and 7B illustrate in a graphical form the effect ofthe invention. In this graph, experimental results are shown for anembodiment as schematically shown in FIG. 2. The horizontal axis denotesthe frequency, whereas in the vertical direction gain (ratio of SO/CS indB) is given (FIG. 7A), and the phase difference. Two lines are drawn,one (the solid line) without use of a compensating actuator, the other(the dotted line) with use of a compensating actuator. Two resonancefrequencies at which the phase lag decreases below −180° are indicatedby peaks 71 a and b at around 1.4 kHz and around 5 kHz. These negativepeaks in the phase cannot be compensated by e.g. simple PID(Proportional Integral Derivative) or PI-lead/lag controllers and thuslimit the bandwidth of the total system. A system with a bandwidth nearthese peaks 71 a and b would be unstable. The dotted line shows that useof the further actuator removes these peaks, and thereby removes theinstabilities. It is noted that, in fact, both of the firstinstabilities are removed with a single actuator. The inventors havefound that, compared to when use is made of an electronic notch filter,the resonance suppressing effect is more stable. Temperature or ageingeffects are smaller. Experiments have also shown that overcompensationdoes not pose a major problem. Overcompensation has an effect as shownin FIGS. 7A and 7B for the second peak b. A phase lag (a negative phasedifference ) is turned into a positive phase difference, which can beseen by the fact that the negative peak (to below −180 degrees) isturned into a positive peak (above the −180° line). In fact, supplyingthe further actuator with an overcompensating signal may beadvantageous, because an added safety margin is then built in againstinstability. This is an advantage of the invention, and the compensatingeffect is robust, especially if a small overcompensation is used.Thermal effects and ageing effects have little influence. The filtersmay be broadband or high-pass filters (which are simple, cheap filters),and the gain g can vary between relatively large margins, while still agood result is achieved.

Addition of the further actuators to the mechanical structure has initself an effect on the resonance frequencies of the mechanicalstructure. Therefore, the filter(s) F are chosen or set to match themechanical structure with further actuators, as is (are) the gain(s).

While the invention has been described in connection with preferredembodiments, it will be understood that modifications thereof within theprinciples outlined above will be evident to those skilled in the art,and that the invention is thus not limited to one or more of thedescribed embodiments but is intended to encompass such modifications.

One such modification is, for instance, an embodiment in which thegain(s) g are tunable (i.e. they have means for setting the gain of thesignal for the further actuator) and the system has means fortemporarily measuring, for instance, the phase lag within a frequencyrange, and retuning the gain in response to the measured phase lag.

The invention is embodied in each new characteristic feature and eachcombination of characteristics features. Any reference signs do notlimit the scope of the claims. Use of the verb “comprise” and itsconjugation does not exclude the presence of elements other than thosestated in a claim. Use of the indefinite article “a” or “an” precedingan element does not exclude the presence of a plurality of suchelements.

Within the concept of the invention a ‘controller means’ is to bebroadly understood and comprise e.g. any piece of hardware (such as acontroller, controller circuit), any circuit or sub-circuit designed forperforming a controlling function as well as any piece of software(computer program or subprogram or set of computer programs, or programcode(s)) designed or programmed to perform a controlling operation inaccordance with the invention as well as any combination of pieces ofhardware and software acting as such, alone or in combination, withoutbeing restricted to the embodiments described.

In summary, the invention may be described as follows.

A lens driving device (1) or an optical read and/or write system,comprises a mechanical structure (3) with an objective lens (2), and anactuator (4, 4′, 6) for controlling the lens position. The lens drivingdevice comprises a further actuator (5, 5 a, 5 b, 5′) on or near themechanical structure so as to at least partially compensate motiongenerated by the first-mentioned actuator (4,6).

1. A lens driving device for an optical read and/or write system,comprising a mechanical structure having an objective lens, and anactuator for controlling the lens position by acting on the mechanicalstructure, characterized in that the lens driving device comprises afurther actuator on or near the mechanical structure for acting on themechanical structure so as to generate at a frequency range a motion ofor in the mechanical structure, to at least partially compensate motiongenerated by the first-mentioned actuator.
 2. A lens driving device asclaimed in claim 1, characterized in that the further actuator isdesigned in such a way that it predominantly excites the resonancefrequency that is to be cancelled.
 3. A lens driving device as claimedin claim 1, characterized in that the actuator comprises apiezo-electric element.
 4. A lens driving device as claimed in claim 1,characterized in that the further actuator comprises a piezo-electricelement.
 5. An optical read and/or write system comprising a lensdriving device comprising a mechanical structure having an objectivelens, and an actuator for controlling the lens position by acting on themechanical structure, the system further comprising a controller meansfor generating a control signal for the actuator, the actuator acting inresponse to the control signal, characterized in that the lens drivingdevice comprises a further actuator on or near the mechanical structurefor acting on the mechanical structure so as to generate at a frequencyrange a motion of or in the mechanical structure, to at least partiallycompensate motion generated by the first-mentioned actuator, thecontroller means comprising means for generating a compensation signalfor said further actuator.
 6. An optical read and/or write system asclaimed in claim 5, characterized in that the further actuator isdesigned in such a way that it predominantly excites the resonancefrequency that is to be cancelled.
 7. An optical read and/or writesystem as claimed in claimed 5, characterized in that the actuatorcomprises a piezo-electric element.
 8. An optical read and/or writesystem as claimed in claimed 5, characterized in that that the furtheractuator comprises a piezo-electric element.